Biomedical subjects

K B Chandran

Publications and source records attributed to K B Chandran.

At least 19 recordsLinked to original sources

PubMed2006

Two-dimensional dynamic simulation of platelet activation during mechanical heart valve closure.

A major drawback in the operation of mechanical heart valve prostheses is thrombus formation in the near valve region. Detailed flow analysis in this region during the valve closure phase is of interest in understanding the relationship between shear stress and platelet activation. A fixed-grid Cartesian mesh flow solver is used to simulate the blood flow through a bi-leaflet mechanical valve employing a two-dimensional geometry of the leaflet with a pivot point representing the hinge region. A local mesh refinement algorithm allows efficient and fast flow computations with mesh adaptation based on the gradients of the flow field in the leaflet-housing gap at the instant of valve closure. Leaflet motion is calculated dynamically based on the fluid forces acting on it employing a fluid-structure interaction algorithm. Platelets are modeled and tracked as point particles by a Lagrangian particle tracking method which incorporates the hemodynamic forces on the particles. A platelet activation model is included to predict regions which are prone to platelet activation. Closure time of the leaflet is validated against experimental studies. Results show that the orientation of the jet flow through the gap between the housing and the leaflet causes the boundary layer from the valve housing to be drawn in by the shear layer separating from the leaflet. The interaction between the separating shear layers is seen to cause a region of intensely rotating flow with high shear stress and high residence time of particles leading to high likelihood of platelet activation in that region.

Algorithms↗

PubMed2006

Comparison of left anterior descending coronary artery hemodynamics before and after angioplasty.

Coronary artery disease (CAD) is characterized by the progression of atherosclerosis, a complex pathological process involving the initiation, deposition, development, and breakdown of the plaque. The blood flow mechanics in arteries play a critical role in the targeted locations and progression of atherosclerotic plaque. In coronary arteries with motion during the cardiac contraction and relaxation, the hemodynamic flow field is substantially different from the other arterial sites with predilection of atherosclerosis. In this study, our efforts focused on the effects of arterial motion and local geometry on the hemodynamics of a left anterior descending (LAD) coronary artery before and after clinical intervention to treat the disease. Three-dimensional (3D) arterial segments were reconstructed at 10 phases of the cardiac cycle for both pre- and postintervention based on the fusion of intravascular ultrasound (IVUS) and biplane angiographic images. An arbitrary Lagrangian-Eulerian formulation was used for the computational fluid dynamic analysis. The measured arterial translation was observed to be larger during systole after intervention and more out-of-plane motion was observed before intervention, indicating substantial alterations in the cardiac contraction after angioplasty. The time averaged axial wall shear stress ranged from -0.2 to 9.5 Pa before intervention compared to -0.02 to 3.53 Pa after intervention. Substantial oscillatory shear stress was present in the preintervention flow dynamics compared to that in the postintervention case.

Angioplasty, Balloon, Coronary↗

PubMed2005

Flow in prosthetic heart valves: state-of-the-art and future directions.

Since the first successful implantation of a prosthetic heart valve four decades ago, over 50 different designs have been developed including both mechanical and bioprosthetic valves. Today, the most widely implanted design is the mechanical bileaflet, with over 170,000 implants worldwide each year. Several different mechanical valves are currently available and many of them have good bulk forward flow hemodynamics, with lower transvalvular pressure drops, larger effective orifice areas, and fewer regions of forward flow stasis than their earlier-generation counterparts such as the ball-and-cage and tilting-disc valves. However, mechanical valve implants suffer from complications resulting from thrombus deposition and patients implanted with these valves need to be under long-term anti-coagulant therapy. In general, blood thinners are not needed with bioprosthetic implants, but tissue valves suffer from structural failure with, an average life-time of 10-12 years, before replacement is needed. Flow-induced stresses on the formed elements in blood have been implicated in thrombus initiation within the mechanical valve prostheses. Regions of stress concentration on the leaflets during the complex motion of the leaflets have been implicated with structural failure of the leaflets with bioprosthetic valves. In vivo and in vitro experimental studies have yielded valuable information on the relationship between hemodynamic stresses and the problems associated with the implants. More recently, Computational Fluid Dynamics (CFD) has emerged as a promising tool, which, alongside experimentation, can yield insights of unprecedented detail into the hemodynamics of prosthetic heart valves. For CFD to realize its full potential, however, it must rely on numerical techniques that can handle the enormous geometrical complexities of prosthetic devices with spatial and temporal resolution sufficiently high to accurately capture all hemodynamically relevant scales of motion. Such algorithms do not exist today and their development should be a major research priority. For CFD to further gain the confidence of valve designers and medical practitioners it must also undergo comprehensive validation with experimental data. Such validation requires the use of high-resolution flow measuring tools and techniques and the integration of experimental studies with CFD modeling.

Animals↗

PubMed2004

Fluid dynamic analysis in a human left anterior descending coronary artery with arterial motion.

A computational fluid dynamic (CFD) analysis is pre sented to describe local flow dynamics in both 3-D spatial and 4-D spatial and temporal domains from reconstructions of intravascular ultrasound (IVUS) and bi-plane angiographic fusion images. A left anterior descending (LAD) coronary artery segment geometry was accurately reconstructed and subsequently its motion was incorporated into the CFD model. The results indicate that the incorporation of motion had appreciable effects on blood flow patterns. The velocity profiles in the region of a stenosis and the circumferential distribution of the axial wall shear stress (WSS) patterns in the vessel are altered with the wall motion introduced in the simulation. The time-averaged axial WSS between simulations of steady flow and unsteady flow without arterial motion were comparable (-0.3 to 13.7 Pa in unsteady flow versus -0.2 to 10.1 Pa in steady flow) while the magnitudes decreased when motion was introduced (0.3-4.5 Pa). The arterial wall motion affects the time-mean WSS and the oscillatory shear index in the coronary vessel fluid dynamics and may provide more realistic predictions on the progression of atherosclerotic disease.

Blood Flow Velocity↗

PubMed2004

Three-dimensional finite element analysis of residual stress in arteries.

Calculation of residual stress in arteries, using the analytical approach has been quite valuable in our understanding of its critical role in vascular mechanics. Stresses are calculated at the central section of an infinitely long tube by imposing a constant axial stretch while deforming the artery from the stress-free state to its unloaded state. However, segments used to perform opening-angle measurements have finite lengths. Further, the stress-free artery configuration is assumed to be circular. Experiments show that they are slightly noncircular. The numerical approach to residual stress calculation can allow us to study both these issues. Using 3D cylindrical geometries and an isotropic material model, we investigated how segment length can affect residual stress calculations and identified the appropriate segment length for experiments. Further, we recorded and used the true noncircular stress-free state of an artery segment, computed the residual stress distribution, and compared it to that from a similar, but circular segment. Our findings suggest that segment length must be ten times the wall thickness for it to be "long" enough. We also found that the circularity assumption may be a reasonable approximation for typical arteries.

Animals↗

PubMed2003

A method for in-vivo analysis for regional arterial wall material property alterations with atherosclerosis: preliminary results.

Atherosclerosis is a diffuse arterial disease developing over many years and resulting in a complicated three-dimensional arterial morphology. The arterial wall material properties have been demonstrated to show regional alterations with atheroma development and growth. We present a mechanical analysis of diseased arterial segments reconstructed from intravascular ultrasound images in order to quantitatively identify regional alterations in the elastic constants with atherosclerotic lesions. We employ a finite element and a displacement sensitivity analysis to divide the arterial segment into regions with different material properties and use an optimization algorithm to identify the elastic constants in these regions. The results with regional variations identified with this method correlated qualitatively with the extent and location of atherosclerotic lesions identified by visual inspection of the affected arteries. The optimized elastic modulus in regions affected by early atherosclerotic lesions ranged from 90.9 to 93.0 kPa where as the corresponding magnitudes in normal arterial segments ranged from 97.9 to 101.0 kPa. This method can be potentially employed to identify the extent and location of atherosclerotic lesions in a systematic analysis and may potentially be used for the early detection of lesion growth.

Animals↗

PubMed2001

Pulsatile flow simulation in arterial vascular segments with intravascular ultrasound images.

Previous studies have indicated a correlation between local variation in wall shear stress in arterial blood flow and atheroma development. The purpose of this study was to analyze the hemodynamics in vascular segments from morphologically realistic three-dimensional (3D) reconstruction, and to compare the computed wall shear stress in a compliant vascular segment model and the corresponding rigid walled model. Cross-sectional images of the segments of femoral and carotid arteries in five Yucatan miniswine were obtained using intravascular ultrasound (IVUS) imaging and the segment geometry was reconstructed at different times in the cardiac cycle. The actual measured wall motion from the reconstruction was employed to specify the moving boundaries for simulation of physiological distensibility. Velocity profiles and wall shear stress were computed using unsteady computational fluid dynamics analysis. The computed results revealed that the maximum wall shear stress in the compliant model was approximately 4-17 percent less than that in the rigid model if the wall motion is larger than 10 percent. Our analysis demonstrates that inaccuracies due to inflow velocity profile can be minimized by the extension of the model upstream. The phase angle between the diameter change and wall shear is affected by the local changes in geometry of the arteries. These simulations can be potentially used to analyze the effect of regional wall motion changes in the presence of atherosclerotic lesions on the local fluid dynamics and to correlate the same with subsequent growth of the lesions.

Animals↗

PubMed2001

Numerical simulation of mechanical mitral heart valve closure.

A computational fluid dynamic simulation of a mechanical heart valve closing dynamics in the mitral position was performed in order to delineate the fluid induced stresses in the closing phase. The pressure and shear stress fields in the clearance region and near the inflow (atrial) side of the valve were computed during the mitral heart valve closure. Three separate numerical simulations were performed. The atrial chamber pressure was assumed to be zero in all the simulations. The first simulation was steady flow through a closed mitral valve with a ventricular pressure of 100 mm Hg (1.3 kPa). In the second simulation, the leaflet remained in the closed position while the ventricular pressure increased from 0 to 100 mm Hg at a rate of 2000 mm Hg/s (simulating leaflet closure by gravity before the ventricular pressure rise - gravity closure). In the third case, the leaflet motion from the fully open position to the fully closed position was simulated for the same ventricular pressure rise (simulating the normal closure of the mechanical valve). Normal closure (including leaflet motion towards closure, and sudden stop in the closed position) resulted in a relatively large negative pressure transient which was not present in the gravity closure simulation. The wall shear stresses near the housing and the leaflet edge close to the inflow side were around 4000 Pa with normal closure compared to about 725 Pa with gravity closure. The large negative pressure transients and significant increase in wall shear stresses due to the simulation of normal closure of the mechanical valve is consistent with the previously reported increased blood damage during the closing phase.

Biomedical Engineering↗

PubMed2000

Numerical study on the effect of secondary flow in the human aorta on local shear stresses in abdominal aortic branches.

Flow in the aortic arch is characterized primarily by the presence of a strong secondary flow superimposed over the axial flow, skewed axial velocity profiles and diastolic flow reversals. A significant amount of helical flow has also been observed in the descending aorta of humans and in models. In this study a computational model of the abdominal aorta complete with two sets of outflow arteries was adapted for three-dimensional steady flow simulations. The flow through the model was predicted using the Navier-Stokes equations to study the effect that a rotational component of flow has on the general flow dynamics in this vascular segment. The helical velocity profile introduced at the inlet was developed from magnetic resonance velocity mappings taken from a plane transaxial to the aortic arch. Results showed that flow division ratios increased in the first set of branches and decreased in the second set with the addition of rotational flow. Shear stress varied in magnitude with the addition of rotational flow, but the shear stress distribution did not change. No regions of flow separation were observed in the iliac arteries for either case. Helical flow may have a stabilizing effect on the flow patterns in branches in general, as evidenced by the decreased difference in shear stress between the inner and outer walls in the iliac arteries.

Algorithms↗

PubMed1998

Numerical study on the effect of steady axial flow development in the human aorta on local shear stresses in abdominal aortic branches.

The three-dimensional flow through a rigid model of the human abdominal aorta complete with iliac and renal arteries was predicted numerically using the steady-state Navier Stokes equations for an incompressible. Newtonian fluid. The model adapted for our purposes was determined from data obtained from cine-CT images taken of a glass chamber that was constructed based on anatomical averages. The iliac arteries had a bifurcation angle of approximately 35 and a branch-to-trunk area ratio of 1.27. whereas the renal arteries had left and right branch angles of 40 and an area ratio of 0.73. The numerical tool FLOW3D (AEA Industrial Technology, Oxfordshire, UK) utilized body-fitted coordinates and a finite volume discretization procedure. Purely axial velocity profiles were introduced at the entrance of the model for a range of cardiac outputs. The four-branch numerical model developed for this investigation produced flow and shear conditions comparable to those found in other reported works. The total wall shear stress distribution in the iliac and renal arteries followed standard trends. with maximum shear stresses occurring in the apex region and lower shear stresses occurring along the lateral walls. Shear stresses and flow rate ratios in the downstream arteries were more effected by inlet Re than the upstream arteries. These results will be used to compare further simulations which take into effect the rotational component of flow which is present in the aortic arch.

Aorta, Abdominal↗

PubMed1997

In vitro identification of angioplasty-induced injury by use of vascular acoustic emissions.

BACKGROUND: We have developed a novel method of diagnosing stress-induced vascular injury. This approach uses the sound energy released from atherosclerotic arterial tissue during in vitro balloon angioplasty to characterize type and severity of induced trauma. METHODS AND RESULTS: Thirty-two postmortem human peripheral arterial specimens 1.0 cm long were subjected to in vitro balloon angioplasty with simultaneous acoustic emission monitoring. Specimens were examined before and after angioplasty to ascertain the extent of angioplasty-induced injury. Gross observation was used to identify dissection. A three-dimensional intravascular ultrasound reconstruction technique was used to estimate the luminal surface area of the specimen. Change in luminal surface area (postangioplasty minus preangioplasty) was used to quantify induced injury. The energy content and spectral distribution of the digitally acquired vascular acoustic emission (VAE) signals were computed. Comparisons of angioplasty-induced trauma with VAE signal characteristics were made. Dissection (mural laceration of variable depth) was observed in 15 of 32 specimens. Eleven showed no evidence of induced dissection, and 6 had preexisting intimal disruptions. The energy content of the VAE signals collected from specimens with dissection was greater than that obtained from those in which dissection was absent: 845 +/- 89.4 mJ (mean +/- SEM; n = 15) versus 128 +/- 40.8 mJ (n = 1 l; P < .001). Comparison of induced trauma and VAE signal energy demonstrated a proportional relationship (r = .87, P < .001, n = 32). CONCLUSIONS: VAE signals contain information characterizing type and severity of angioplasty-induced arterial injury. Because vascular injury is related to adverse procedural outcome, development of VAE technology as an adjunct to conventional diagnostic modalities may facilitate optimal balloon angioplasty delivery and postprocedural care.

Analysis of Variance↗

PubMed1997

Regional vascular mechanical properties by 3-D intravascular ultrasound with finite-element analysis.

A method employing intravascular ultrasound (IVUS) and simultaneous hemodynamic measurements, with resultant finite element analysis (FEA) of accurate three-dimensional IVUS reconstructions (3-DR), was developed to estimate the regional distribution of arterial elasticity. Human peripheral arterial specimens (iliac and femoral, n = 7) were collected postmortem and perfused at three static transmural pressures: 80, 120, and 160 mmHg. At each pressure, IVUS data were collected at 2.0-mm increments through a 20.0-mm segment and used to create an accurate 3-DR. Mechanical properties were determined over normotensive and hypertensive ranges. An FEA and optimization procedure was implemented in which the elemental elastic modulus was scaled to minimize the displacement error between the computer-predicted and actual deformations. The "optimized" elastic modulus (Eopt) represents an estimate of the component element material stiffness. A dimensionless variable (beta), quantifying structural stiffness, was computed. Eopt of nodiseased tissue regions (n = 80) was greater than atherosclerotic regions (n = 88) for both normotensive (Norm) and hypertensive (Hyp) pressurization: Norm, 9.3 +/- 0.98 vs. 3.5 +/- 0.30; Hyp, 11.3 +/- 0.72 vs. 8.5 +/- 0.47, respectively (mean +/- SE x 10(6) dyn/cm2; P < 0.01 vs. nondiseased). No differences in beta between nondiseased and atherosclerotic tissue were noted at Norm pressurization. With Hyp pressurization, beta of atherosclerotic regions were greater than nondiseased regions: 21.5 +/- 2.21 vs. 14.0 +/- 2.11, respectively (P < 0.03). This method provides a means to identify regional in vivo variations in mechanical properties of arterial tissue.

Arteriosclerosis↗

PubMed1996

Computation of vascular flow dynamics from intravascular ultrasound images.

Analysis of three-dimensional velocity profiles and wall shear stress distribution in a segment of an artery reconstructed from in vivo imaging data are presented in this study. Cross-sectional images of a segment of the abdominal aorta in dogs were obtained using intravascular ultrasound (IVUS) imaging employing a constant pull back technique. Simultaneous measurement of pressures distal and proximal to the vessel segment along with gated pulsed Doppler velocity measurements were also obtained. The three-dimensional geometry of the vascular segment was reconstructed from the IVUS images during peak forward flow phase, and a computational mesh was constructed from the data. A quasi-steady analysis of incompressible Newtonian fluid was performed with a finite difference general purpose computational analysis program FLOW3D. The velocity at the inlet and pressure at the outlet measured at the corresponding time (time referenced to ECG) were used to specify the boundary conditions for the computational flow model. The computed results compared favorably with previously reported results. The purpose of the present study was to analyze the hemodynamics in vascular segments from morphologically realistic three-dimensional reconstructions. The method can be potentially employed in analyzing the hemodynamics in the region of atherosclerotic plaques at various stages of development and the reactivity of the vessel in response to pharmacological and mechanical interventions.

Animals↗

PubMed1996

Cavitation dynamics of medtronic hall mechanical heart valve prosthesis: fluid squeezing effect.

The cause of cavitation in mechanical heart valves is investigated with Medtronic Hall tilting disk valves in an in vitro flow system simulating the closing event in the mitral position. Recordings of pressure wave forms and photographs in the vicinity of the inflow surface of the valve are attempted under controlled transvalvular loading rates averaged during valve closing period. The results revealed presence of a local flow field with a very high velocity around the seat stop of mechanical heart valves that could induce pressure reduction below liquid vapor pressure and a cloud of cavitation bubbles. The analysis of the results indicates that the "fluid squeezing" between the stop and occluder as the main cause of cavitation in Medtronic Hall valves. The threshold loading rate for cavitation initiation around the stop was found to be very low (300 and 400 mmHg/s; half the predicted normal human loading rate that was estimated to be 750 mmHg/s) because even a mild impact created a high speed local flow field on the occluder surface that could induce pressure reduction below vapor pressure. The present study suggests that mechanical heart valves with stops at the edge of major orifice region are more vulnerable to cavitation, and hence, have higher potential for damage on valve components and formed elements in blood.

Blood Flow Velocity↗

PubMed1996

Pressure distribution near the occluders and impact forces on the outlet struts of Björk-Shiley convexo-concave valves during closing.

BACKGROUND AND AIMS OF THE STUDY: An in vitro study of the mechanics of closure of Björk-Shiley convexo-concave (BSCC) valves is presented in order to investigate the mechanics of outlet strut fracture reported in a small fraction of the implanted valves. MATERIALS AND METHODS: Four BSCC 29 mm valves instrumented with strain gages on the outlet strut legs were mounted in the mitral position of an axisymmetric flow chamber of a mock pulsatile flow loop. Measurements of the pressure field in the vicinity of the occluder, closing velocity of the occluder tip in the major orifice, and the impact force between the occluder and outlet strut at the instant of valve closure were obtained at a range of physiologic flow rates. RESULTS: The results indicated an uneven pressure distribution on the occluder associated with a tendency for the occluder to over-rotate and induce loads on the outlet struts. The impact loads on the outlet struts were asymmetric with load on one leg being larger than the other by up to 25%. These results are consistent with single leg separation preceding outlet strut fracture in most of the valve failures reported. Orientation of the valve with respect to the mitral orifice (major orifice towards the top or bottom) did not significantly affect the loads on the outlet strut. A significant variation in the impact loads of the four valves was measured for identical experimental conditions suggesting that valve specific factors influence outlet strut loads. CONCLUSIONS: This study provided an understanding of the cause-effect relationship between valve dynamics and outlet strut fracture.

Biomechanical Phenomena↗

PubMed1996

Effect of valve holder flexibility on cavitation initiation with mechanical heart valve prostheses: an in vitro study.

BACKGROUND AND AIM OF THE STUDY: Several in vitro studies have reported on cavitation bubble visualization with mechanical heart valves and the cavitation intensity has been correlated with the design of the valve, the load on the valve occluder, the velocity of the occluder tip, and the negative pressure transients in the vicinity of the occluder. These studies demonstrated the presence of cavitation for certain types of valves under simulated normal physiological loading conditions. However, extrapolation of these results to cavitation initiation in vivo has been questioned due to limitations of the in vitro studies in simulating the in vivo tissue compliance. The present study was intended to analyze the effect of valve holder flexibility (simulating compliance of the suture ring and the surrounding tissue in vivo) on cavitation dynamics. METHODS: Cavitation bubbles were visualized on three types of mechanical heart valves (Medtronic Hall, Edwards-Duromedics, and CarboMedics) in our in vitro set up, and pressure transients were measured close to the occluder at valve closure. Two different flexible valve holders made of Teflon (elastic modulus, E = 400 MPa) and low density polyethylene (E = 180 MPa) were employed and the results were compared with those with a rigid Plexiglas holder (E = 2930 MPa) of the same geometry (3'' by 3'' wide and 1/8'' thick). RESULTS: Significant reductions were noted in the intensity of cavitation bubbles appearing along the clearance region of the Medtronic Hall and CarboMedics valves, with increasing valve holder flexibility. However, no attenuation was observed for the bubbles appearing around the seating lip or stop of the Edwards-Duromedics and Medtronic Hall valves that are believed to be caused by the fluid squeezing effect. CONCLUSIONS: The results of the study suggest that timing of the mechanism to initiate cavitation is a critical factor in cavitation attenuation with flexible valve holders. If cavitation is initiated before the flexible valve holder responds to the impact at valve closure (such as due to squeeze film effect), cavitation intensity remains unchanged. Based on the results of the study, we propose that tissue compliance in vivo may not attenuate cavitation initiation for certain types of mechanical heart valves depending on the cavitation initiation mechanism.

Biomechanical Phenomena↗

PubMed1996

Variability of regurgitation in Björk-Shiley mitral valves and relationship to disc occluder design: an in vitro two-dimensional color-Doppler flow mapping study.

BACKGROUND AND AIMS OF THE STUDY: Normal prosthetic valves have regurgitation that varies according to valve type and design. The Björk-Shiley prosthetic mitral valve is a tilting disc valve that has undergone design changes since its introduction. From 1969 to 1981, Delrin, was used to make the disc occluder. After 1971, the occluder was made from Pyrolite (i.e. Conical and Radiopaque-Spherical valves). Our aim was to quantify the regurgitation of Delrin and Radiopaque-Spherical Björk-Shiley prosthetic mitral valves with color-Doppler flow mapping in an in vitro model that simulates transesophageal echocardiography imaging. MATERIALS AND METHODS: Normal unimplanted Björk-Shiley Delrin (BSD), Björk-Shiley Radiopaque-Spherical (BSS) and explanted (17 +/- 3 yrs) BSD valves (25, 27, and 29 mm) were studied in a pulse duplication system. The regurgitant leakage volume of the valves was measured with an electromagnetic flow probe at flow rates of 3.0, 5.0, and 7.0 L/min, a pulse rate of 70 beats/min, and a mean systemic pressure of 100 mmHg. Color-Doppler flow mapping was performed with a 3.7 MHz transducer positioned on the atrial chamber at an image depth of eight centimeters. The maximal regurgitant jet areas were measured offline and averaged from three beats. RESULTS: Maximal jet area, measured with color-Doppler flow mapping, correlated with regurgitant leakage volume (r = 0.82). Normal unimplanted and explanted BSD valves had greater regurgitant leakage volumes and jet areas than BSS valves for all sizes and flow rates studied. Regurgitant jet areas of normal unimplanted and explanted BSD valves were similar. CONCLUSION: Knowledge of the type of Björk-Shiley valve is important in the clinical evaluation of regurgitation severity by transesophageal echocardiography. The echocardiographic appearance of regurgitation of BSD valves does not necessarily imply valve dysfunction.

Blood Flow Velocity↗

PubMed1995

Numerical simulation of instantaneous backflow through central clearance of bileaflet mechanical heart valves at closure: shear stress and pressure fields within clearance.

Instantaneous backflow through central clearance of bileaflet heart valves at the instant of closure is investigated. An Edwards-Duromedics valve in the mitral position is employed to measure the transient pressures near the entrance and exit region in an in vitro flow chamber. A region surrounding the clearance is modelled, and two-dimensional quasi-steady-state numerical simulations are performed, with the measured transient pressure difference across the clearance as a driving force for the flow. The results show that pressure difference several times larger than the driving pressure used to close the valve is established across the clearance for about 0.5 ms at the moment of closure. The resulting average wall shear stress is an order of magnitude larger than the turbulent Reynolds stresses reported distal to the valve during opening. A local jump in the shear stress distribution and fall in the pressure distribution are observed at the entrance region. Rounding of the corners in the channel entrance attenuates these spikes. The results of the study indicate that backflow through clearance at closure may be one reason for the haemolysis and thrombosis associated with mechanical heart valves, despite the short duration of the flow field.

Heart Valve Prosthesis↗